Methods for driving electro-optic displays

ABSTRACT

There are provided methods for driving an electro-optic display having a plurality of display pixels, a such method includes receiving an image, converting the image into a YCbCr image; and processing the YCbCr image to generate a luma image. The method further includes calculating variations in a local area for the YCbCr image to obtain a variation map, and calculating an effect ratio map using the calculated variation.

REFERENCE TO RELATED APPLICATIONS

This application is related to and claims priority to U.S. ProvisionalApplication 63/122,936 filed on Dec. 8, 2020.

The entire disclosures of the aforementioned application is hereinincorporated by reference.

SUBJECT OF THE INVENTION

This invention relates to methods for driving electro-optic displays.More specifically, this invention relates to driving methods forrendering images on electro-optic displays with color filters or colorfilter arrays.

BACKGROUND

Electro-optic displays typically have a backplane provided with aplurality of pixel electrodes each of which defines one pixel of thedisplay; conventionally, a single common electrode extending over alarge number of pixels, and normally the whole display is provided onthe opposed side of the electro-optic medium. The individual pixelelectrodes may be driven directly (i.e., a separate conductor may beprovided to each pixel electrode) or the pixel electrodes may be drivenin an active matrix manner which will be familiar to those skilled inbackplane technology. One way to achieve color in electro-optic displaysis to equip such displays with a color filter array (CFA).

However, CFA based displays, including both emissive and reflectivedisplays, suffer from loss of color spatial resolution due to subpixels.Typical CFA displays have red, green, and blue filters. Therefore, ifone of the primary colors is shown on a display, it has only one thirdor less (less because there is filling between subpixels) of the displayarea to be utilized. Where one pixel in a source image corresponds toone pixel in a display where each pixel location has one of the colorfilters. In a simple rendering process, if a given pixel location has ared filter, only red channel value will be taken from the same pixel inthe source image. The same goes for green and blue filters. This cansometimes lead to a loss of color fine details such as colored finetexts. And this issue can become more severe when a display's colorgamut is small, dynamic range is low, or display resolution is low.

As such, driving methods that preserves color fine details in CFAdisplays are needed.

SUMMARY OF INVENTION

Accordingly, in one aspect, the subject matter presented herein providesfor a method for driving an electro-optic display having a plurality ofdisplay pixels, the method can include receiving an image, convertingthe image into a YCbCr image, and processing the YCbCr image to generatea luma image.

In some embodiments, the step of processing the YCbCr image to generatea luma image may further include boosting outputs from a red channel, agreen channel, and a blue channel. And boosting the outputs from the redchannel, the green channel, and the blue channel may include matchingthe luma to that of a target pixel.

In some other embodiments, the method may further include calculatingvariations in a local area for the YCbCr image to obtain a variationmap, where calculating the variations may include calculating thevariations for each of the red channel, green channel, and the bluechannel of the YCbCr image, and calculating the variations comprisesmaximizing variations for each of the red channel, green channel, andthe blue channel of the YCbCr image.

In some embodiments, the method may further include calculating aneffect ratio map using the calculated variation, where calculating aneffect ratio map may include taking pixel values from the luma image.

In some other embodiments, calculating an effect ratio map may includetaking pixel values from the received image.

In yet another embodiment, an electro-optic display configured to carryout the method may include a color filter array. In some embodiments,the display may further include an electrophoretic material comprising aplurality of electrically charged particles disposed in a fluid andcapable of moving through the fluid under the influence of an electricfield. In some other embodiments, the electrically charged particles andthe fluid are confined within a plurality of capsules or microcells. Inyet another embodiment, the electrically charged particles and the fluidare present as a plurality of discrete droplets surrounded by acontinuous phase may include a polymeric material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram representing an electrophoretic display;

FIG. 2 shows a circuit model of the electro-optic imaging layer;

FIG. 3 illustrates a cross sectional view of an electro-optic displayhaving a colored filter array;

FIG. 4 is a block diagram illustrating a driving method in accordancewith the subject matter disclosed herein;

FIG. 5 illustrates an exemplary process flow for rendering a color imagefor a CFA display;

FIG. 6 illustrates a variance map in accordance with the subject matterdisclosed herein; and

FIG. 7 illustrates an effect ratio map in accordance with the subjectmatter disclosed herein.

DETAILED DESCRIPTION

The present invention relates to methods for driving electro-opticdisplays, especially bistable electro-optic displays, and to apparatusfor use in such methods. More specifically, this invention relates todriving methods which may allow for reduced “ghosting” and edge effects,and reduced flashing in such displays. This invention is especially, butnot exclusively, intended for use with particle-based electrophoreticdisplays in which one or more types of electrically charged particlesare present in a fluid and are moved through the fluid under theinfluence of an electric field to change the appearance of the display.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms “black” and “white” may be usedhereinafter to refer to the two extreme optical states of a display, andshould be understood as normally including extreme optical states whichare not strictly black and white, for example, the aforementioned whiteand dark blue states. The term “monochrome” may be used hereinafter todenote a drive scheme which only drives pixels to their two extremeoptical states with no intervening gray states.

Some electro-optic materials are solid in the sense that the materialshave solid external surfaces, although the materials may, and often do,have internal liquid- or gas-filled spaces. Such displays using solidelectro-optic materials may hereinafter for convenience be referred toas “solid electro-optic displays”. Thus, the term “solid electro-opticdisplays” includes rotating bichromal member displays, encapsulatedelectrophoretic displays, microcell electrophoretic displays andencapsulated liquid crystal displays.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

The term “impulse” is used herein in its conventional meaning of theintegral of voltage with respect to time. However, some bistableelectro-optic media act as charge transducers, and with such media analternative definition of impulse, namely the integral of current overtime (which is equal to the total charge applied) may be used. Theappropriate definition of impulse should be used, depending on whetherthe medium acts as a voltage-time impulse transducer or a charge impulsetransducer.

Much of the discussion below will focus on methods for driving one ormore pixels of an electro-optic display through a transition from aninitial gray level to a final gray level (which may or may not bedifferent from the initial gray level). The term “waveform” will be usedto denote the entire voltage against time curve used to effect thetransition from one specific initial gray level to a specific final graylevel. Typically such a waveform will comprise a plurality of waveformelements; where these elements are essentially rectangular (i.e., wherea given element comprises application of a constant voltage for a periodof time); the elements may be called “pulses” or “drive pulses”. Theterm “drive scheme” denotes a set of waveforms sufficient to effect allpossible transitions between gray levels for a specific display. Adisplay may make use of more than one drive scheme; for example, theaforementioned U.S. Pat. No. 7,012,600 teaches that a drive scheme mayneed to be modified depending upon parameters such as the temperature ofthe display or the time for which it has been in operation during itslifetime, and thus a display may be provided with a plurality ofdifferent drive schemes to be used at differing temperature etc. A setof drive schemes used in this manner may be referred to as “a set ofrelated drive schemes.” It is also possible, as described in several ofthe aforementioned MEDEOD applications, to use more than one drivescheme simultaneously in different areas of the same display, and a setof drive schemes used in this manner may be referred to as “a set ofsimultaneous drive schemes.”

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R. A., et al., “Video-SpeedElectronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003).It is shown in U.S. Pat. No. 7,420,549 that such electro-wettingdisplays can be made bistable.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thesepatents and applications include:

(a) Electrophoretic particles, fluids and fluid additives; see forexample U.S. Pat. Nos. 7,002,728 and 7,679,814;

(b) Capsules, binders and encapsulation processes; see for example U.S.Pat. Nos. 6,922,276 and 7,411,719;

(c) Microcell structures, wall materials, and methods of formingmicrocells; see for example U.S. Pat. Nos. 7,072,095 and 9,279,906;

(d) Methods for filling and sealing microcells; see for example U.S.Pat. Nos. 7,144,942 and 7,715,088;

(e) Films and sub-assemblies containing electro-optic materials; see forexample U.S. Pat. Nos. 6,982,178 and 7,839,564;

(f) Backplanes, adhesive layers and other auxiliary layers and methodsused in displays; see for example U.S. Pat. Nos. 7,116,318 and7,535,624;

(g) Color formation and color adjustment; see for example U.S. Pat. Nos.7,075,502 and 7,839,564.

(h) Applications of displays; see for example U.S. Pat. Nos. 7,312,784;8,009,348;

(i) Non-electrophoretic displays, as described in U.S. Pat. No.6,241,921 and U.S. Patent Application Publication No. 2015/0277160; andapplications of encapsulation and microcell technology other thandisplays; see for example U.S. Patent Application Publications Nos.2015/0005720 and 2016/0012710; and

(j) Methods for driving displays; see for example U.S. Pat. Nos.5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997; 6,753,999;6,825,970; 6,900,851; 6,995,550; 7,012,600; 7,023,420; 7,034,783;7,061,166; 7,061,662; 7,116,466; 7,119,772; 7,177,066; 7,193,625;7,202,847; 7,242,514; 7,259,744; 7,304,787; 7,312,794; 7,327,511;7,408,699; 7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251;7,602,374; 7,612,760; 7,679,599; 7,679,813; 7,683,606; 7,688,297;7,729,039; 7,733,311; 7,733,335; 7,787,169; 7,859,742; 7,952,557;7,956,841; 7,982,479; 7,999,787; 8,077,141; 8,125,501; 8,139,050;8,174,490; 8,243,013; 8,274,472; 8,289,250; 8,300,006; 8,305,341;8,314,784; 8,373,649; 8,384,658; 8,456,414; 8,462,102; 8,537,105;8,558,783; 8,558,785; 8,558,786; 8,558,855; 8,576,164; 8,576,259;8,593,396; 8,605,032; 8,643,595; 8,665,206; 8,681,191; 8,730,153;8,810,525; 8,928,562; 8,928,641; 8,976,444; 9,013,394; 9,019,197;9,019,198; 9,019,318; 9,082,352; 9,171,508; 9,218,773; 9,224,338;9,224,342; 9,224,344; 9,230,492; 9,251,736; 9,262,973; 9,269,311;9,299,294; 9,373,289; 9,390,066; 9,390,661; and 9,412,314; and U.S.Patent Applications Publication Nos. 2003/0102858; 2004/0246562;2005/0253777; 2007/0070032; 2007/0076289; 2007/0091418; 2007/0103427;2007/0176912; 2007/0296452; 2008/0024429; 2008/0024482; 2008/0136774;2008/0169821; 2008/0218471; 2008/0291129; 2008/0303780; 2009/0174651;2009/0195568; 2009/0322721; 2010/0194733; 2010/0194789; 2010/0220121;2010/0265561; 2010/0283804; 2011/0063314; 2011/0175875; 2011/0193840;2011/0193841; 2011/0199671; 2011/0221740; 2012/0001957; 2012/0098740;2013/0063333; 2013/0194250; 2013/0249782; 2013/0321278; 2014/0009817;2014/0085355; 2014/0204012; 2014/0218277; 2014/0240210; 2014/0240373;2014/0253425; 2014/0292830; 2014/0293398; 2014/0333685; 2014/0340734;2015/0070744; 2015/0097877; 2015/0109283; 2015/0213749; 2015/0213765;2015/0221257; 2015/0262255; 2016/0071465; 2016/0078820; 2016/0093253;2016/0140910; and 2016/0180777.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned 2002/0131147. Accordingly, for purposes of thepresent application, such polymer-dispersed electrophoretic media areregarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display.” In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, e.g., a polymeric film. See, forexample, International Application Publication No. WO 02/01281, andpublished U.S. Application No. 2002/0075556, both assigned to SipixImaging, Inc.

Many of the aforementioned E Ink and MIT patents and applications alsocontemplate microcell electrophoretic displays and polymer-dispersedelectrophoretic displays. The term “encapsulated electrophoreticdisplays” can refer to all such display types, which may also bedescribed collectively as “microcavity electrophoretic displays” togeneralize across the morphology of the walls.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R. A., et al., “Video-SpeedElectronic Paper Based on Electrowetting,” Nature, 425, 383-385 (2003).It is shown in copending application Ser. No. 10/711,802, filed Oct. 6,2004, that such electro-wetting displays can be made bistable.

Other types of electro-optic materials may also be used. Of particularinterest, bistable ferroelectric liquid crystal displays (FLCs) areknown in the art and have exhibited remnant voltage behavior.

Although electrophoretic media may be opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, some electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, the patents U.S.Pat. Nos. 6,130,774 and 6,172,798, and 5,872,552; 6,144,361; 6,271,823;6,225,971; and 6,184,856. Dielectrophoretic displays, which are similarto electrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode.

A high-resolution display may include individual pixels which areaddressable without interference from adjacent pixels. One way to obtainsuch pixels is to provide an array of non-linear elements, such astransistors or diodes, with at least one non-linear element associatedwith each pixel, to produce an “active matrix” display. An addressing orpixel electrode, which addresses one pixel, is connected to anappropriate voltage source through the associated non-linear element.When the non-linear element is a transistor, the pixel electrode may beconnected to the drain of the transistor, and this arrangement will beassumed in the following description, although it is essentiallyarbitrary and the pixel electrode could be connected to the source ofthe transistor. In high-resolution arrays, the pixels may be arranged ina two-dimensional array of rows and columns, such that any specificpixel is uniquely defined by the intersection of one specified row andone specified column. The sources of all the transistors in each columnmay be connected to a single column electrode, while the gates of allthe transistors in each row may be connected to a single row electrode;again the assignment of sources to rows and gates to columns may bereversed if desired.

The display may be written in a row-by-row manner. The row electrodesare connected to a row driver, which may apply to a selected rowelectrode a voltage such as to ensure that all the transistors in theselected row are conductive, while applying to all other rows a voltagesuch as to ensure that all the transistors in these non-selected rowsremain non-conductive. The column electrodes are connected to columndrivers, which place upon the various column electrodes voltagesselected to drive the pixels in a selected row to their desired opticalstates. (The aforementioned voltages are relative to a common frontelectrode which may be provided on the opposed side of the electro-opticmedium from the non-linear array and extends across the whole display.As in known in the art, voltage is relative and a measure of a chargedifferential between two points. One voltage value is relative toanother voltage value. For example, zero voltage (“OV”) refers to havingno voltage differential relative to another voltage.) After apre-selected interval known as the “line address time,” a selected rowis deselected, another row is selected, and the voltages on the columndrivers are changed so that the next line of the display is written.

However, in use, certain waveforms may produce a remnant voltage topixels of an electro-optic display, and as evident from the discussionabove, this remnant voltage produces several unwanted optical effectsand is in general undesirable.

As presented herein, a “shift” in the optical state associated with anaddressing pulse refers to a situation in which a first application of aparticular addressing pulse to an electro-optic display results in afirst optical state (e.g., a first gray tone), and a subsequentapplication of the same addressing pulse to the electro-optic displayresults in a second optical state (e.g., a second gray tone). Remnantvoltages may give rise to shifts in the optical state because thevoltage applied to a pixel of the electro-optic display duringapplication of an addressing pulse includes the sum of the remnantvoltage and the voltage of the addressing pulse.

A “drift” in the optical state of a display over time refers to asituation in which the optical state of an electro-optic display changeswhile the display is at rest (e.g., during a period in which anaddressing pulse is not applied to the display). Remnant voltages maygive rise to drifts in the optical state because the optical state of apixel may depend on the pixel's remnant voltage, and a pixel's remnantvoltage may decay over time.

As discussed above, “ghosting” refers to a situation in which, after theelectro-optic display has been rewritten, traces of the previousimage(s) are still visible. Remnant voltages may give rise to “edgeghosting,” a type of ghosting in which an outline (edge) of a portion ofa previous image remains visible.

An Exemplary EPD

FIG. 1 shows a schematic of a pixel 100 of an electro-optic display inaccordance with the subject matter submitted herein. Pixel 100 mayinclude an imaging film 110. In some embodiments, imaging film 110 maybe bistable. In some embodiments, imaging film 110 may include, withoutlimitation, an encapsulated electrophoretic imaging film, which mayinclude, for example, charged pigment particles.

Imaging film 110 may be disposed between a front electrode 102 and arear electrode 104. Front electrode 102 may be formed between theimaging film and the front of the display. In some embodiments, frontelectrode 102 may be transparent. In some embodiments, front electrode102 may be formed of any suitable transparent material, including,without limitation, indium tin oxide (ITO). Rear electrode 104 may beformed opposite a front electrode 102. In some embodiments, a parasiticcapacitance (not shown) may be formed between front electrode 102 andrear electrode 104.

Pixel 100 may be one of a plurality of pixels. The plurality of pixelsmay be arranged in a two-dimensional array of rows and columns to form amatrix, such that any specific pixel is uniquely defined by theintersection of one specified row and one specified column. In someembodiments, the matrix of pixels may be an “active matrix,” in whicheach pixel is associated with at least one non-linear circuit element120. The non-linear circuit element 120 may be coupled betweenback-plate electrode 104 and an addressing electrode 108. In someembodiments, non-linear element 120 may include a diode and/or atransistor, including, without limitation, a MOSFET. The drain (orsource) of the MOSFET may be coupled to back-plate electrode 104, thesource (or drain) of the MOSFET may be coupled to addressing electrode108, and the gate of the MOSFET may be coupled to a driver electrode 106configured to control the activation and deactivation of the MOSFET.(For simplicity, the terminal of the MOSFET coupled to back-plateelectrode 104 will be referred to as the MOSFET's drain, and theterminal of the MOSFET coupled to addressing electrode 108 will bereferred to as the MOSFET's source. However, one of ordinary skill inthe art will recognize that, in some embodiments, the source and drainof the MOSFET may be interchanged.)

In some embodiments of the active matrix, the addressing electrodes 108of all the pixels in each column may be connected to a same columnelectrode, and the driver electrodes 106 of all the pixels in each rowmay be connected to a same row electrode. The row electrodes may beconnected to a row driver, which may select one or more rows of pixelsby applying to the selected row electrodes a voltage sufficient toactivate the non-linear elements 120 of all the pixels 100 in theselected row(s). The column electrodes may be connected to columndrivers, which may place upon the addressing electrode 106 of a selected(activated) pixel a voltage suitable for driving the pixel into adesired optical state. The voltage applied to an addressing electrode108 may be relative to the voltage applied to the pixel's front-plateelectrode 102 (e.g., a voltage of approximately zero volts). In someembodiments, the front-plate electrodes 102 of all the pixels in theactive matrix may be coupled to a common electrode.

In some embodiments, the pixels 100 of the active matrix may be writtenin a row-by-row manner. For example, a row of pixels may be selected bythe row driver, and the voltages corresponding to the desired opticalstates for the row of pixels may be applied to the pixels by the columndrivers. After a pre-selected interval known as the “line address time,”the selected row may be deselected, another row may be selected, and thevoltages on the column drivers may be changed so that another line ofthe display is written.

FIG. 2 shows a circuit model of the electro-optic imaging layer 110disposed between the front electrode 102 and the rear electrode 104 inaccordance with the subject matter presented herein. Resistor 202 andcapacitor 204 may represent the resistance and capacitance of theelectro-optic imaging layer 110, the front electrode 102 and the rearelectrode 104, including any adhesive layers. Resistor 212 and capacitor214 may represent the resistance and capacitance of a laminationadhesive layer. Capacitor 216 may represent a capacitance that may formbetween the front electrode 102 and the back electrode 104, for example,interfacial contact areas between layers, such as the interface betweenthe imaging layer and the lamination adhesive layer and/or between thelamination adhesive layer and the backplane electrode. A voltage Viacross a pixel's imaging film 110 may include the pixel's remnantvoltage.

In use, it is desirable for an electro-optic display as illustrated inFIGS. 1 and 2 to update to a subsequent image without flashing thedisplay's background. However, the straightforward method of using anempty transition in image updating for a background color to backgroundcolor (e.g., white-to-white, or black-to-black) waveform may lead to thebuild-up of edge artifacts (e.g., bloomings). In a black and whiteelectro-optic display, the edge artifacts may be reduced by usingspecialized waveforms such as a top off waveform. However, in anelectro-optic display such as an electrophoretic display (EPD) withcolors generated using a color filter array (CFA), maintaining colorquality and contrast may be challenging sometimes.

FIG. 3 illustrates a cross sectional view of a CFA based colored EPD inaccordance with the subject matter disclosed herein. As shown in FIG. 3,a color electrophoretic display (generally designated 300) comprising abackplane 302 bearing a plurality of pixel electrodes 304. To thisbackplane 302 may be laminated an inverted front plane laminate, thisinverted front plane laminate may comprise a monochrome electrophoreticmedium layer 306 having black and white extreme optical states, anadhesive layer 308, a color filter array 310 having red, green and blueareas aligned with the pixel electrodes 304, a substantially transparentconductive layer 312 (typically formed from indium-tin-oxide, no) and afront protective layer 314.

In practice, variations in local areas of an image may be used topreserve fine color details of a CFA display. The subject matterpresented herein utilizes Local Variation-based Subpixel rendering, orLVS rendering. A process where the use of color variation in local areasof a given input image, and then determine if this area is a detailpreserving area, is adopted to better presented the fine color details.Referring now to FIG. 4, where an exemplary method 400 for driving a CFAdisplay is presented in accordance with the subjected disclosed herein.

In some embodiments, a LVS rendering algorithm may firstly take a sourceimage (e.g., a sRGB image or img_sRGB) and a subpixel location map(e.g., imMASK) that defines which pixel location has which color filteras the input. Subsequently, at step 402 of FIG. 4, the sRGB image may beconverted to a YCbCr image using methods commonly adopted in theindustry, such as a linear transformation defined in ITU-RRecommendation BT.601.

Next, at step 404 of FIG. 4, a luma image (e.g., img_luma) may bedefined according to an exemplary algorithm presented below:

for k=1:3

-   -   img_luma(imMASK==k)=img_Y(imMASK==k)*c_boost_RGB(k)

end

where img_Y is a Y-channel image from YCbCr image, c_boost_RGB is a listof three coefficients to boost red, green, and blue channel outputs. Theboosting may be ideal to match luma of a target pixel since transparencyis different among three channels. The coefficients are tunableparameters designed to balance the image brightness. As illustrated inFIG. 6.

After the creation of the luma image, in step 406, local variation maybe calculated to generate a variation map of the image (See FIG. 6).Calculation of the variation may be done in local areas for eachchannels in YCbCr. For the purpose of illustration, a local area size of3×3 pixel area is used herein, for example, as illustrated in FIG. 5. Anexemplary algorithm for generating a variation map is illustrated here:

For each channel in YCbCr:

-   -   For each local area:        -   calculate mean pixel value,        -   For each pixel:            -   subtract the mean,            -   take absolute value,            -   take square-root,        -   End        -   calculate mean of the square-rooted values of all pixels.            This is conceptually the        -   variation in a local area.    -   End

End

For each pixel:

-   -   Take maximum variation among three channels

End

In some embodiments, for each channel in YCbCr and for each local areaas defined above, one may calculate a mean pixel value by subtract themean from each pixel, take an absolute value, and then take square-rootof that value. Where pixel value may be defined as a value describinghow bright a pixel is, and/or what color it should be. In the simplestcase of binary images, the pixel value may be a 1-bit number indicatingeither foreground or background. For grayscale images, a pixel value maybe a single number that represents the brightness of the pixel. Forexample, for a byte image, this number may be stored as an 8-bit integergiving a range of possible values from 0 to 255, where zero is taken tobe black, and 255 is taken to be white. Values in between make up thedifferent shades of gray. To represent color images, separate red, greenand blue components may be specified for each pixel, and so the pixelvalue may actually be a vector of three numbers. Often the threedifferent components may be stored as three separate grayscale imageknown as color planes (one for each of red, green and blue), which haveto be recombined when displaying or processing. Subsequently thevariation in a local area may be calculated by calculate the mean of thesquare-root values of all the pixels in the local area.

In alternative embodiments, instead of taking square-root of absolutedifference between pixel value and neighboring average, one can alsotake standard deviation, variance, or any other means to define localvariations. Similarly, when pooling variations among three channelstogether, one can take any form such as mean and median instead of max.The variations could be calculated in three channels together instead ofcalculating them for each channel.

Next, in step 408, an effect ratio map may be generated, as shown inFIG. 7, the effect ratio map configured to define detail preservationeffect to each display pixels. The effect ratio for a given pixel isdefined in a piece-wise linear function illustrated below:

r=(v−k1)/(k2−k1)

-   -   r=1 if r>1    -   r=0 if r<0        where r is an effect ratio, v is a variation calculated above,        and k1 and k2 are tunable parameters.

If it's full effect (r=1), the pixel value is taken from img_luma at thespecified pixel location. If it's no effect (r=0), the pixel value istaken from a corresponding color channel in img_sRGB at the specifiedpixel location. Pixel values are linearly interpolated if the effect isbetween 0 and 1.

Note that the effect ratio map can be calculated in any linear ornon-linear functions taking variations as input.

It will be apparent to those skilled in the art that numerous changesand modifications can be made to the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. A method for driving an electro-optic display having a plurality ofdisplay pixels, the method comprising: receiving an image; convertingthe image into a YCbCr image; and processing the YCbCr image to generatea luma image.
 2. The method of claim 1, wherein the step of processingthe YCbCr image to generate a luma image further comprising boostingoutputs from a red channel, a green channel, and a blue channel.
 3. Themethod of claim 2, wherein boosting the outputs from a red channel, agreen channel, and a blue channel comprises matching the luma to that ofa target pixel.
 4. The method of claim 1 further comprising calculatingvariations in a local area for the YCbCr image to obtain a variationmap.
 5. The method of claim 4 wherein calculating the variationscomprises calculating the variations for each of the red channel, greenchannel, and the blue channel of the YCbCr image.
 6. The method of claim5 wherein calculating the variations comprises maximizing variations foreach of the red channel, green channel, and the blue channel of theYCbCr image.
 7. The method of claim 4 further comprising calculating aneffect ratio map using the calculated variation.
 8. The method of claim7 wherein calculating an effect ratio map comprises taking pixel valuesfrom the luma image.
 9. The method of claim 7 wherein calculating aneffect ratio map comprises taking pixel values from the received image.10. An electro-optic display configured to carry out the method of claim1 further comprising a color filter array.
 11. The electro-optic displayaccording to claim 10 comprising an electrophoretic material comprisinga plurality of electrically charged particles disposed in a fluid andcapable of moving through the fluid under the influence of an electricfield.
 12. The electro-optic display according to claim 10 wherein theelectrically charged particles and the fluid are confined within aplurality of capsules or microcells.
 13. The electro-optic displayaccording to claim 10 wherein the electrically charged particles and thefluid are present as a plurality of discrete droplets surrounded by acontinuous phase comprising a polymeric material.
 14. A displaycontroller capable of controlling the operation of a bistableelectro-optic display, the controller configured to carry out a drivingmethod for operating the display, the method comprises: receiving animage; converting the image into a YCbCr image; and processing the YCbCrimage to generate a luma image.
 15. The controller according to claim14, wherein the driving method further comprising calculating variationsin a local area for the YCbCr image to obtain a variation map.
 16. Thecontroller according to claim 15, wherein the driving method furthercomprising calculating an effect ratio map using the calculatedvariation.
 17. The controller according to claim 15, wherein the drivingmethod further comprising calculating an effect ratio map comprisestaking pixel values from the luma image.